This invention relates to a device for adjusting the opening travel of the movable plate of a moulding machine for plastic materials, by means of which it is possible to stop said plate in a pre-established position during said opening travel.
As it is well-known, the moulding presses or moulding machines for plastic materials, in particular thermoplastic materials, are provided with a mould holder unit comprising a pair of plates between which is positioned the mould and one of which is arranged to move axially with respect to the other, departing from or approaching said other plate in order to allow, respectively, the opening or the closure of the mould.
Normally, said movable plate is actuated by a fluid operated cylinder controlling a kinematic toggle mechanism, which is apt to cause said plate to move from a first to a second position, in which positions said plate is located, respectively, at the minimal and the maximal distance from said other plate.
The movable plate reaches the first of said two positions when the kinematic toggle mechanism is in its extended position, i.e. in a position in which some of the members which constitute said mechanism arrange themselves in such a way as to originate substantially a chain of support bars which are able to support the high injection pressures which discharge themselves onto the movable plate.
The travel of this plate, which is necessary for moving it from said first to said second position, must be adjustable in accordance with the characteristics and dimensions of the mould mounted on the press, so that the above-mentioned second position must be variable to conform with the different conditions of operation of the machine.
In the prior art machines, the adjustment of the travel of said plate, and consequently the fixing of said second position, is obtained by acting on the hydraulic device which controls the fluid operated cylinder actuating the kinematic toggle device; in fact, by discontinuing the feed of the working fluid to the fluid operated cylinder during the opening travel of the movable plate it is possible to stop the latter in a pre-established position. However, the adjustment of the end of travel position of the movable plate, effected in the way described above, presents some disadvantages. First of all, said position cannot be fixed very precisely, nor is it possible to stop said plate always in the same position during each opening cycle of the machine. Furthermore, the stop of the movable plate thus obtained is not positive, that is to say that if external forces of a high value act on the plate, said plate does not maintain its position.
Also known are manually adjustable screw devices for adjusting the travel of such plates. Such devices comprise substantially a threaded stem which is screwed in a nut rigidly connected to the frame of the machine and whose one end passes through a wall of the casing of the fluid operated cylinder to act as a stop for the piston of said cylinder; the position in which said piston and, consequently, the movable plate are stopped can be varied by turning the threaded stem.
The devices of this type present the disadvantage of having a very high axial dimension; in fact, said stem, which protrudes axially from the end of the fluid operated cylinder, being provided with a smooth cylindrical portion which is apt to traverse the end wall of the casing of said cylinder, has a length which is at least twice as long as the length of the travel of the piston which it has to stop.